Error performance of terrestrial free space optical links with subcarrier time diversity

In this work, the performance of binary phase shift keying (BPSK) subcarrier intensity-modulated terrestrial free-space optical (FSO) communication link is analysed in the presence of noise and atmospheric turbulence. To ameliorate channel fading caused by the atmospheric turbulence, we propose a subcarrier time delay diversity (STDD) scheme in which the delayed versions of the original data are re-transmitted on different subcarriers. In short range links, a single re-transmission scheme results in an estimated gain of up to 4.5 dB in received irradiance. With sufficient link margins, short range FSO links might be able to cope without the use of any diversity techniques but for links spanning over 1 km, turbulence induced fading effect must be compensated for. In this study, the authors show that the subcarrier STDD is a viable scheme for this purpose.